docs: def nonanalytical_func(x, a, b):
      """
      This can be any pythonic function which should be fitted, typically one
      which is not easily written or supported as an analytical expression.
      """
      # Do your non-trivial magic here. In this case a Piecewise, although this
      # could also be done symbolically.
      y = np.zeros_like(x)
      y[x > b] = (a * (x - b) + b)[x > b]
      y[x <= b] = b
      return y